Laser annealing systems are utilized in the production of advanced display technologies such as liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs). One of the challenges associated with laser annealing is that the illumination field of a laser beam generally has a profile that includes inhomogeneities. Such inhomogeneities can have various causes including, for example, local defects like absorption or scattering centers in optical components, modulations in the raw laser beam and interference or diffraction from beam homogenizer arrays.
FIG. 1 illustrates a prior art laser annealing system 10. The system 10 includes a cylindrical lens (not shown) for focusing a laser beam 16, such as an excimer laser beam, onto the surface of an a-silicon-coated substrate 12. The illumination field 14 of the laser beam 16 typically has dimensions up to 470 mm (y-direction) by 0.4 mm (x-direction). This elongated beam spot is often referred to as a line focus.
The system 10 processes a substrate by scanning a single track in the x-direction and then, if the substrate's dimension in y-direction is longer than the corresponding dimension of the illumination field of the laser beam 16, shifting in the y-direction to scan a subsequent track.
In a typical system, the substrate is carried past the beam spot in the x-direction by a highly accurate translation stage. The scan velocity of the stage is on the order of six millimeters per second. Assuming a laser pulse repetition rate of 300 Hertz, the substrate will move about 20 microns in the x-direction between shots. This means that about 15 to 20 pulses (with an overlap of about 95%) will hit the substrate at each position.
FIG. 2 illustrates a magnified view of part of a substrate 20 that has been treated with the prior art laser annealing system of FIG. 1. As can be seen in FIG. 2, inhomogeneities in a laser beam across the long axis can manifest themselves as parallel lines or stripes running in the x-direction, which is the scanning direction of the laser beam. Width w represents a typical size of inhomogeneities in the illumination field of a laser beam, and the width of the illumination field is much larger than w.
Conventional beam homogenization techniques have focused on improving the homogeneity of the laser beam. Some prior art systems use diffusers in the laser beam path to improve the homogeneity of the laser beam. Other prior art systems have used light tunnels, lenses, lens arrays diffractive optical elements (DOE) or holographs to improve homogeneity. Prior art homogenization techniques cannot, however, remove all small-scale inhomogeneities. As a result, stripes such as those illustrated in FIG. 2 still occur. To address small-scale inhomogeneities, some prior art systems have used rotating wedges to move the laser beam in a circular motion on the object being scanned. Such systems tend to average out small-scale inhomogeneities, but these systems are not suitable for use with laser annealing systems.